In the case of applications common in measuring and control technology, e.g. for inspection, control, and/or automation of complex processes, a multiplicity of transmitters, e.g. pressure, temperature, flow and/or fill level transmitters, are commonly in use.
A transmitter is usually composed of a transducer that registers a physical quantity and converts it to an electrical quantity, and an electronics that converts the electrical quantity into a measurement signal.
These transmitters frequently have an electrical current output, i.e. the transmitter regulates a current, incoming and outgoing, on a conductor pair, as a function of an instantaneous, measured value of the physical quantity. The measurement signal is a signal current in the case of these transmitters. In accordance with a standard common in measuring and control technology, the signal current is set as a function of the instantaneous, measured value to values between a minimum signal current of 4 mA and a maximum signal current of 20 mA.
The measurement signals are usually collected by a superordinated unit, e.g. a control and/or regulating unit. The superordinated unit supplies, as a function of the instantaneous measured values, display, control and/or regulating signals for the inspection, control and/or automation of a process. Examples of this are programmable logic controllers (PLC), process control systems (PCS) or personal computers (PC).
In the case of conventional transmitters, an ohmic measuring resistor is inserted in the conductor pair of the electrical current output. A voltage drop across this resistance is proportional to the instantaneous signal current, and can thus be referenced in the transmitter for regulating the signal current.
Although the use of such a measuring resistor is widespread, it is accompanied by some disadvantages. Often occurring over the conductor pair is not only the signal transmission, but also the supplying of power for the transmitter. For the transmitter with a limited supply voltage, e.g. 12 V, and a signal current determined by the present measured value, e.g. a current between 4 mA and 20 mA, only a limited amount of power is available for use. This small power is additionally reduced by the power loss through the resistance. Thus, even less power is available to the transmitter.
The power loss at the resistor is converted to heat. This heat is unwanted, especially in the case of operation where there is danger of explosion.
In order that a high measuring accuracy can be achieved, the resistance of the measuring resistor must be correspondingly large. The larger the resistance, however, the more power is lost at the sensor resistor and the more heat is formed.
A further disadvantage is that resistances of resistors are temperature-dependent. This temperature-dependence can lead to additional measuring errors.